Ceramic fibers, such as silicon carbide fibers, are used as reinforcement for ceramic- and metal-matrix composites. The fibers enhance toughness, strength, stiffness, and creep resistance in the composites. Silicon carbide fiber is desirable as a reinforcing fiber because it is predicted to have high elastic modulus and high tensile strength.
Several methods are known in the art for making silicon carbide (SiC) fibers. For example, U.S. Pat. No. 5,279,780 issued to Lipowitz et al. on Jan. 18, 1994, discloses a near-stoichiometric polycrystalline silicon carbide fiber that can withstand temperatures up to 1,400.degree. C. while maintaining a useful strength. The process comprises heating an amorphous or microcrystalline ceramic fiber containing silicon, carbon, and oxygen in an environment comprising a volatile sintering aid. The sintering aid was placed in the furnace and allowed to volatilize in the presence of the fiber during pyrolysis.
U.S. Pat. No. 5,366,943 issued to Lipowitz et al. on Nov. 24, 1994, discloses a process for preparing a polycrystalline silicon carbide fiber that can withstand temperatures up to 1,400.degree. C. The process comprises heating an amorphous or microcrystalline ceramic fiber containing silicon, carbon, and at least one element selected from the group consisting of titanium, zirconium, and chlorine in an environment comprising a volatile sintering aid. The volatile sintering aid can be a compound of iron, magnesium, lithium, beryllium, boron, aluminum, thorium, yttrium, lanthanum, cerium, and mixtures thereof. The sintering aid is volatilized by placing solid or liquid sintering aid in the furnace with the ceramic fiber during heating.
However, in these processes the sintering aid is placed in the furnace and allowed to volatilize when the fiber is heated. The amount of volatile sintering aid in the furnace depends on the vapor pressure of the sintering aid at the temperature in the furnace. This can be undesirable when boron oxide is the sintering aid. If too much boron oxide is present, the boron oxide can oxidize silicon carbide, thereby weakening the polycrystalline silicon carbide fiber produced.
Therefore, one object of this invention is to provide a process for preparing silicon carbide fiber using a controlled concentration of boron oxide vapor at, or any level below, saturation in the furnace. The concentration of boron oxide vapor in the furnace is controlled by producing the boron oxide vapor in situ by the reaction of a boron containing material with an oxidizing agent. The rate of addition of the oxidizing agent is varied to produce the controlled concentration of boron oxide vapor.